1. Field of the Invention
The invention pertains to the field of variable cam timing systems. More particularly, the invention pertains to a variable cam timing system having a reservoir of pooled oil to prevent air from entering the chambers of the phaser.
2. Description of Related Art
In a variable cam timing (VCT) system, the timing gear on the camshaft is replaced by a variable angle coupling known as a “phaser”, having a rotor connected to the camshaft and a housing connected to (or forming) the timing gear, which allows the camshaft to rotate independently of the timing gear, within angular limits, to change the relative timing of the camshaft and crankshaft. The term “phaser”, as used here, includes the housing and the rotor, and all of the parts to control the relative angular position of the housing and rotor, to allow the timing of the camshaft to be offset from the crankshaft. In any of the multiple-camshaft engines, it will be understood that there would be one phaser on each camshaft, as is known to the art.
FIGS. 1A and 1B show a conventional oil pressure actuated phaser. In an oil pressure actuated (OPA) phaser, engine oil pressure is applied to a chamber 2, 12 on one side of the vane 6 or the other. Oil from the opposing chamber 2, 12 is exhausted back to the oil sump through lines 8, 10. The applied engine oil pressure alone is used to move the vane 6 in the advancing or retarding direction. Engine oil to the chambers 2, 12 is controlled by a centrally located spool valve 4. The spool valve 4 is comprised of a spool 9 with cylindrical lands 9a, 9b and is surrounded by a cylindrical sleeve 13. The spool 9 is biased by a spring on one side and actuator on the other side (not shown).
FIG. 1A shows the OPA phaser in an advance position when torque reversals are not present. Oil 5 flows from the retard chamber 12 through line 10 and out to the oil sump (not shown). Supply 18 provides oil 5 to the advance chamber 2 through line 8.
FIG. 1B shows the OPA phaser advancing when torque reversals 20 are present. Oil 5 is fed from the supply 18 to the advancing chamber 2 through line 8, moving the vane 6 in the direction shown by the arrow. Oil 5 exits the retard chamber 12 through line 10. When a torque reversal 20 occurs air 19 within the cylindrical sleeve 13 housing the spool 9, is drawn into line 10 by a vacuum created by the torque reversal 20. The air 19 travels through line 10 to the retard chamber 12 and eventually accumulates in the chamber 12 to a point where severe aeration occurs and the phaser experiences a large amount of oscillation and may totally lose its phasing capability. The same accumulation may occur when the phaser was retarding.
The accumulation of air in the chambers as described above would also occur in a single check valve torsion assist (TA) phaser or a two check valve torsion assist (TA) phaser.
Various patents have tried to decrease or prevent air from entering the hydraulic chambers. U.S. Pat. No. 5,803,029 discloses a helical spline phaser where torque fluctuations are dampened between the camshaft and pulleys by the oil retained in the delay hydraulic chamber and the advance chamber. When changing cams, the first and second oil lines of the control valve are shut off to the advance and delay oil passages. All of the oil discharged from the oil pump is fed to the valve lift control mechanism.
JP6093815A2 discloses discharge ports that communicate with an oil discharge preventing passage, which extends above the hydraulic chamber. The position of the discharge preventing passage above the hydraulic chamber air is prevented from flowing into the hydraulic chamber.
JP07224616 discloses helical spline phaser in which a ring gear present between the timing pulley housing and the camshaft that prevents air from entering the advance or retard chamber in which oil is not present.